1. Field of the Invention
This invention relates to a semiconductor light emitting device made of nitride compound semiconductors.
2. Description of the Related Art
GaN and other nitride compound semiconductors have recently been remarked as materials of light emitting diodes or semiconductor lasers for emitting blue to ultraviolet light. For example, gallium nitride compound semiconductors, having a direct transition band structure, are recognized to exhibit a high emission efficiency. Additionally, blue semiconductor laser diodes and semiconductor light emitting diodes using these compounds are expected to be applicable as light sources for high-density information processing or other various display light sources because of their short pulsation wavelengths.
In this application, the term "nitride compound semiconductor" pertains to any III-V compound semiconductor expressed as B.sub.x In.sub.y Al.sub.z Ga.sub.(1-x-y-z) N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, 0.ltoreq.z.ltoreq.1) containing phosphorus (P), arsenic (As) and/or others in addition to N as group-V elements, and also pertains to any II-IV-N.sub.2 -type compound such as BeCN.sub.2.
In order to induce pulsation of a semiconductor laser with a low threshold value or to induce emission of a light emitting diode with a high luminance, it is important to suppress the flow of a current into a region not contributing to emission of light. For example, in semiconductor lasers, laser pulsation does not occur unless the current is supplied into a bottle-neck portion in the current layer. In light emitting diodes, by suppressing the flow of a current into a luminous region blocked by a wire bonding or another electrode, a certain luminance is obtained with a low current.
In this respect, various proposals have been made on current blocking structures. Among them, Japanese Patent Laid-Open Publications Nos. 4-242985 and 4-213878 disclose semiconductor lasers in which a stripe-shaped electrode is in direct contact with GaN compound crystal. In these structures, since a p-type layer with a relatively high resistance contacts with the electrode with a high contact resistance over small area, the resistance between the p-type electrode and the p-type layer is large, and this results in increasing the resistance and the operation voltage of the entire device. Therefore, an internal current blocking structure permitting a wide extension of the p-type layer is required to decrease the operation voltage.
Also on the problem, some proposals have been made. Japanese Patent Laid-Open Publication No 8-18159 teaches hydrogen implantation to make carriers inactive. Mere implantation, however, causes damage to the crystal, and annealing or other processing must be done to recover the crystallographic structure. However, such processing causes the implanted hydrogen to separate from the crystal, thereby permits the carriers to restore activity, and fails to effectuate the current confining function.
Japanese Patent Laid-Open Publications Nos. 8-46291, 8-88441, 8-107247 and 8-111558 teach current blocking structures made by forming in a p-type crystal layer a portion of a different conduction type or an insulating layer such as SiO.sub.2 film. These techniques rely on selectively etching a grown crystal. GaN compound crystal, however, is difficult to etch by a wet method using a solution, and compels one to use dry etching by various gases. Dry etching, however, causes damage to the surface of a non-etched region, and hence causes a current leakage. The current leakage makes current injection into the active layer inefficient, and therefore disturbs laser pulsation under a low threshold current density.
That is, any conventional structure made by using dry etching or ion implantation, damages to crystals may cause a current leakage. Such current leakage makes current injection into the active layer inefficient, which results in preventing laser pulsation under a low threshold current density or realization of a highly efficient light emitting diode.
On the other hand, in light emitting devices using nitride compound semiconductors, n-type layers and p-type layers are grown on a sapphire substrate in this order, interposing a buffer layer between them, and the electrode to the n-type layer is formed after removing the p-type layers by etching. There are various proposals on the electrode to the n-type layers. For example, Japanese Patent Laid-Open Publication No. 3-252175 discloses aluminum (Al). The use of aluminum, however, invites decomposition or deterioration during various annealing steps after deposition of the electrode, and degrades wire bonding.
Japanese Patent Laid-Open Publication No. 7-240508 teaches stacking gold (Au) on an Al electrode to facilitate wire bonding. However, the Inventors have experimentally recognized that deterioration or decomposition still remains unsolved because Au and Al are mixed or oxidized during annealing. As a countermeasure against this problem, insertion of Ti as a barrier metal is disclosed in Japanese Patent Laid-Open Publication No 8-274372. However, the Inventors have experimentally found that this approach certainly made wire bonding possible, but invited an increase in contact resistance of the electrode.
That is, conventional electrode structure of nitride compound semiconductor light emitting devices fails to decrease the contact resistance and cannot ensure good wire bonding.